What is the shape of a catenary locus ?

Hello.
It's been over 30 years since my days at the university and have trouble thinking...

A parabola reflects all the axial rays converging to a focus. A focus is a point. A single point. Not a locus area.
If the reflector is a catenary, where do the axial rays converge to ?
Cannot be a point as the curve is different. What is its shape ? Is it a line, is it a circle, is it an ellipse locus ?

If the above is not clear enough; in other words :

A catenary reflector has a focal area -locus- instead of a focus point as a parabolic reflector has.

Is it a circle ? Is it a line ?

The intention is to place a pipe of the same section and shape as the locus at the locus of a catenary reflector trough, to be insolated and boil water circulating in it.
The catenary lenght (span) is 4 feet.
What would be the correct catenary depth for the locus to be centered between the ends of the catenary ?

Thanks,
Miguel